Linear solenoids are electromechanical devices that convert electrical energy into a linear mechanical motion, which is used to control electrical, pneumatic or hydraulic systems. Solenoids are used in valves, relays and contactors.
Electromechanical solenoids consist of an electromagnetically inductive coil that is wound to encircle a movable steel or iron slug, termed “the armature” or “plunger.” The coil is shaped such that the plunger can be moved in and out of its center, altering the inductance of the coil. The plunger is used to provide a mechanical force to activate the control mechanism, for example opening and closing of a valve.
A solenoid coil needs a higher current during activation, called the pull—in current, to pull the plunger into the solenoid. However, once the plunger has moved completely, the solenoid coil needs only approximately 30% of its nominal current, called “the hold current,” to keep the plunger in the same position. DC solenoids having coils that operate continuously at their nominal current, which is limited by the resistance of the coil, will have an increase in temperature of the coil due to the higher power dissipation. Once the complete plunger movement is detected, the steady-state current can be reduced to the hold current to minimize the power consumption in the solenoid. The detection of the plunger movement is required in safety-critical applications to detect proper operation of the valve, relays or contactors. Movement of the plunger can be slow, due to factors such as friction, rusting and other mechanical impediments to the movement of the plunger.
FIG. 1 shows an example of a known solenoid drive circuit, shown generally as 100. A DC input voltage at 102 is applied to one terminal of a solenoid coil 104, the other terminal of the solenoid coil is connected to a transistor 108, controlling current through the solenoid, which is sensed by sense resistor RSENSE 110. Transistor 108 is controlled by current controlled solenoid driver 112, which will drive the solenoid at its nominal current until the plunger has moved completely, at which time the current can be reduced to its hold value. Freewheeling diode 106 is used to eliminate the sudden voltage spike seen across the transistor when it is switched off by the current controlled solenoid driver.
FIG. 2 shows the known excitation current waveform of a solenoid, generally as 200. As soon as the solenoid is energized at 202, the current begins to increase as shown at 204. When the current reaches IPEAK at 206, the plunger starts moving because of the sufficient magnetic field created by the solenoid coil. The movement of the plunger induces back EMF in the coil, and hence, the solenoid current starts dropping. At 208, the plunger has moved completely and the current dips to IVALLEY. After the plunger strokes, the current continues on its normal upward path, as shown at 210, to its maximum value, as shown at 212, which is limited by the resistance of the coil. The prominent dip in the excitation curve from IPEAK to IVALLEY is an indication of plunger movement.
One known plunger position sensing method includes hall sensors to detect the position of the plunger. The mechanical mounting of these sensors are complex and their performance is affected by ageing and external field. In addition, the hall sensor will provide a signal at the end of the plunger movement, and therefore, cannot detect slow movement of the plunger.
Other plunger movement detection logic uses fixed references for detecting peak and valley current, or utilize algorithmic solutions. These algorithms may fail during temperature variation or during slow movement of plunger.
There is a need for a simple, low-cost and reliable technique for detecting complete solenoid movement that can detect plunger movement over wide variation of temperature and also detect slow-moving plungers.